The present invention relates to chopper control equipment for electric railcars, which smoothly and accurately controls the regenerative braking by compensating for the differences in characteristics of a plurality of motors, when the speed of d-c shunt motors in an electric railcar is to be controlled by relying upon the chopper control of the armature voltage or armature current of the motors and relying upon a field exciting device.
Owing to the development in semiconductor elements, most of the electric railcars have been controlled by relying upon d-c chopper circuits. In particular, high sticking performance is required between the wheels and the rails for the electric locomotives. For this purpose, an attempt has been made to use d-c shunt motors and drive motors and to connect a plurality of motors in parallel. Moreover, it has been attempted to separately control the currents that flow into the armatures which are connected in parallel by independent armature chopper means, i.e., by controlling the armature voltage or the armature current by chopper circuits, so that stickiness of the wheels to the rails can be individually controlled even when some wheels undergo racing on the rails, in order to minimize the decrease of tractive force as a whole.
With the shunt motors, however, the field current characteristics change little with changes in the speed. Therefore, if different characteristics are imparted to the motors during their manufacture, greatly different armature currents flow even at the same speed. To control the field currents for a plurality of motors, therefore, the differences in the armature currents must be compensated by the above-mentioned armature chopper. This means that the controllable range of the armature chopper is narrowed. In particular, an armature chopper which is not set to a minimal conduction ratio in a high-speed regenerative braking region means that the armature voltage is decreased, and the kinetic energy possessed by the electric railcar is not fully recovered. This problem can be solved if the currents flowing into the shunt windings of the motors are separately controlled. This, however, necessitates exciting devices for each of the motors, and results in increased size, increased weight, and increased manufacturing costs.
FIG. 1 shows regenerative brake-notch curves obtained by a conventional control system, in which curves A and B represent characteristics when the same current If is allowed to flow into motors having difference characteristics so as to operate them at a minimum conduction ratio (0.1 in this case) of the armature choppers. For the purpose of easy understanding, FIG. 1 illustrates the case when there are two electric motors. Since the matter resides in the differences in the characteristics, no problem arises even if the motors are represented by only two motors. It will be seen that the armature current IaA and IaB differ greatly even at the same speed V.sub.1, and the braking forces to be born therefore differ greatly. Namely, the wheel tends to skid on the rail in the case of the curve B and, furthermore, problems arise with regard to the heat that is generated in the motor since a large armature current flows.